1. Field of the Invention
The Invention relates generally to devices for electrical and mechanical installation of conductive terminal pins or pigtails into and through conductive back-planes, and more specifically, to conductive bushings for providing the electrical and mechanical interconnection of electrical contacts to a selected one of a plurality of superimposed insulated metallic plates.
2. Description of the Prior Art
Modern electronic hardware is produced in highly automated fabrication and assembly operations. Printed circuit boards, separately manufactured, are assembled in connector arrangements in circuit combinations, for digital computers and the like, by suitably interconnecting groups of printed circuit boards. In such arrangements, it is conventional to plug each printed circuit board into a connector mounted on the front face of a metal plate (back-plane) which serves to establish a ground plane for the electrical circuits on the printed circuit boards, as well as providing mechanical support for the assembled printed circuit boards and connectors. In addition to a ground plane, a separate, generally parallel, second conductive plane operates as a power distribution buss or voltage plane. These are separated by a layer of insulating material forming a sandwich of the two conductive planes separated by the said insulating material layer.
The connector assemblies into which the circuit boards are plugged include elongated conductive pins (also variously called pins, tails, pigtails, etc.) which electrically extend contacts of the circuit board connector sockets through the ground and voltage back-planes to project beyond for further connection. This further connection frequently is accomplished by a known technique, generally referred to as wire-wrapping. For that, a square tail is commonly employed.
In the usual prior art arrangement of the type aforementioned, a bushing of one form or another is used to mechanically and electrically secure at least selected ones of said tails within a corresponding opening either in the ground or voltage plane, as appropriate. Prior art bushings used for that purpose have generally been of the machined type, including external knurling or the like, for press-fitting into the corresponding back-plane bore.
A typical prior art arrangement to which the present invention is applicable is shown in U.S. Pat. No. 3,518,610. The so-called insert or bushing of the machine type, is shown in U.S. Pat. Nos. 2,995,617; 3,365,539; 3,760,496; 3,704,441; and in the technical literature in general.
The type of progressive die formation of sheet metal parts applicable to manufacturing of the novel bushing in accordance with the present invention, is shown in U.S. Pat. No. 3,288,915. Wirewrapping, per se, is illustrated in U.S. Pat. Nos. 3,209,311; 3,420,087, and 3,365,539.
Other types of bushings, including sheet metal types are shown in U.S. Pat. Nos. 3,420,087; 2,967,285; and 3,564,479. These other types of bushings rely in general on the characteristics of resilient sheet metal (spring metal) for their griping action within a back-plane bore or hole and also against the pin or tail inserted therethrough.
The aforementioned machined ground bushing has a high spring rate (virtually a press-fit or interference fit). This imposes a close tolerance requirement on the bushing, the plate bore and the terminal post. Tolerance variations may result in a requirement for hand-fitting which greatly increases the cost of production. Moreover, machine bushings are poorly adapted to automated assembly, whereas progressively formed sheet metal parts may be automatically inserted while still joined to a strip of metal from which they were blanked and formed.
The nature of a machined bushing for the present purpose usually requires that it be elongated beyond the thickness of the metallic back-plane into which it is inserted. This is because the resilient members gripping the pin or tail passing therethrough must ordinarily be beyond the relatively monolithic bushing body which is press-fit into the back-plane which it is intended to contact. This has the effect of requiring a larger clearance hole in the back-plane of the sandwiched pair through which it must pass without contact. Accordingly, extra manufacturing processing is required in order to accommodate both sizes of back-plane bores (holes).
The various prior art sheet metal bushings encountered are costly and either poorly adapted to the double plane (sandwich arrangement aforementioned) configuration, poorly adapted to automatic insertion, or leave much to be desired in the quality of the electrical and/or mechanical retention of the pin or tail passing therethrough.
The manner in which the present invention deals with the prior art disadvantages will be evident as this description proceeds.